Early this morning, Nigeria’s President Umaru Yar’Adua returned–via ambulance–to his home in his nation’s capital, 3 months and one day after he left Nigeria for medical treatment in Saudi Arabia.

According to Presidential spokesman Olusegun Adeniyi, although Mr. Yar’Adua’s health has improved, “while the president completes his recuperation, Vice President Jonathan will continue to oversee the affairs of state.”

Nigeria’s parliament voted to officially recognize Mr. Jonathan as Acting President on February 10, 2010. That vote specified that Mr. Jonathan would cede power to Mr. Yar’Adua once he was medically fit to resume leading the country.

Mr. Yar’Adua has not made any public appearances since he left Nigeria on November 23, 2009, and he has not spoken in public since a BBC radio interview from a hospital bed on January 13, 2009. (That BBC interview served as the president’s official communication that he was not well enough to rule–the basis for the legislatures’ recognition of Mr. Jonathan as Acting President.)

In response to the news of Yar’Adua’s return, U.S. Assistant Secretary of State Johnnie Carson welcomed him back but expressed concern about Nigeria’s future:

“Recent reports … continue to suggest that President Yar’Adua’s health remains fragile and that he may still be unable to fulfill the demands of his office….We hope that President Yar’Adua’s return to Nigeria is not an effort by his senior advisers to upset Nigeria’s stability and create renewed uncertainty in the democratic process.”

Read more in the New York Times, NPR and BBC.